I found what appears to be an exact match of this lovely little bone and brass folding ruler that dates it to 1820 or so; I don't know for certain, but definitely 19th century, and most definitely a really wonderful little object. The numbering on the inches is pretty faint at this point but can still be read, as can the lines dividing each inch into eighths. A fantastic gift for an architect or anyone who frequently takes the measure of things, though once you've held this in your hand it may be fairly hard to part with!
The bone on one half is a bit darker/more orange in tone that the other, which I think makes it feel extra special. I have not touched the brass but it appears to be in good condition and would likely polish up if desired. It opens smoothly and closes tight, with a little pin near one end that meets the tiny hole on the other side perfectly. 6" long x 3/8" x 1/8" when open; 3 3/16" x 7/16" x 1/8" when closed. Photos are shot with 2-3 times magnification.